A potential performance slowdown could be our geometry shader using
extra logic in packing varyings. Test this by adding a variable
number of varyings to the point sprites benchmark.

Initial tests show it could slow down the test by about 30-40% or so,
stil not catastrophic compared to D3D9.

BUG=angle:705

Change-Id: Ia815f0ee28e2af2d3dc4d9f5675c27bbeb4cb119
Reviewed-on: https://chromium-review.googlesource.com/216468Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>